The present invention relates to a continuous process for the preparation of nitrobenzene by the adiabatic nitration of benzene with a mixture of sulfuric and nitric acids (so-called ‘mixed acid’). Such a process was first claimed in U.S. Pat. No. 2,256,999 and is described in more modern embodiments in U.S. Pat. No. 4,091,042, U.S. Pat. No. 5,313,009 and U.S. Pat. No. 5,763,697.
A common feature of the adiabatic processes described is that the starting materials, benzene and nitric acid, are reacted in a large excess of sulfuric acid, which absorbs the heat of reaction evolved and the water formed in the reaction.
The reaction procedure generally involves combining the nitric acid and sulfuric acid to give so-called ‘nitrating acid’ (also called ‘mixed acid’). Benzene is metered into this nitrating acid. The reaction products are essentially water and nitrobenzene. In the nitration reaction, benzene is used in at least the stoichiometric amount, but preferably in 2% to 10% excess, based on the molar amount of nitric acid. In accordance with the state of the art, the crude nitrobenzene formed in the reaction apparatuses and separated from the acid phase in the phase separation apparatus is washed and worked up by distillation, as described e.g. in EP 1 816 117 B1 (page 2, lines 26 to 42), U.S. Pat. No. 4,091,042 (cf. above) or U.S. Pat. No. 5,763,697 (cf. above). A characteristic feature of this work-up is that, after washing, unreacted excess benzene is separated from nitrobenzene in a final distillation and re-used in the nitration reaction as recycle benzene, which also comprises low-boiling non-aromatic organic compounds (so-called ‘low boilers’) (cf DE 10 2009 005 324 A1). The treatment of the off-gas from the adiabatic nitration reaction is described in EP 0 976 718 B1. The off-gas from the acid circuit and from the final crude nitrobenzene is drawn off, combined and passed through a NOx absorber to recover dilute nitric acid, which is recycled into the reaction. The sulfuric acid, referred to as recycle acid, is concentrated in a flash evaporator and freed of organics as far as possible. Traces of high-boiling organics, e.g. nitrobenzene, dinitrobenzene and nitrophenols, remain in the recycle acid and are thus also recycled into the reaction.
The quality of an adiabatic process for the nitration of aromatic hydrocarbons is defined on the one hand by the content of unwanted reaction by-products in the product, which are formed by multiple nitration or oxidation of the aromatic hydrocarbon or the nitroaromatic. In the preparation of nitrobenzene one strives to minimize the content of dinitrobenzene and nitrophenols, particularly trinitrophenol (picric acid), which is classified as explosive.
The quality of an adiabatic process is defined on the other hand by use of the smallest possible amount of energy to prepare the nitrobenzene. This is assured inter alia by utilizing the adiabatic heat of reaction to concentrate the sulfuric acid or by minimizing the stoichiometric excess of benzene required in the reaction, based on nitric acid, in order not to expend unnecessary amounts of energy on the washing and final distillation of the crude nitrobenzene.
To obtain nitrobenzene with particularly high selectivities, the nature of the mixed acid to be used has been stipulated in detail (EP 0 373 966 B1, EP 0 436 443 B1 and EP 0 771 783 B1) and it has been indicated that the content of by-products is determined by the value of the maximum temperature (EP 0 436 443 B1, column 15, lines 22 to 25). It is also known that a high initial conversion is advantageous for a high selectivity and that this is achieved when optimum intermixing is applied at the start of the reaction (EP 0 771 783 B1, paragraphs [0012] to [0014]).
Outstanding selectivities are obtained when the chosen initial reaction temperature is very low (WO 2010/051616 A1), although this is tantamount to increasing the reaction time several fold. A high space-time yield is advantageous for the industrial application of a process since this makes it possible to construct compact reaction equipment distinguished by low capital expenditure in relation to capacity. This procedure is therefore counterproductive.
As regards the quality of the benzene starting material in the adiabatic preparation of nitrobenzene, EP 2 246 320 A1 describes that, depending on its source, commercially available benzene can be contaminated to a greater or lesser extent. Typical impurities are other aromatics, especially toluene and xylene, which can each be present in amounts of up to 0.05 wt % in benzene of common purity. Other impurities typical for benzene are anon-aromatic organic compounds, which can account for a total of up to 0.07 wt %. Cyclohexane (up to 0.03 wt %) and methylcyclohexane (up to 0.02 wt %) are cited in particular here. In the concentrations mentioned, the impurities described above have either no or only a very slight adverse effect on the subsequent steps of the MDI process chain (MDI=di- and polyisocyanates of the diphenylmethane series), e.g. by slightly disturbing the waste water and off-gas treatment in the nitrobenzene process due to non-aromatic organics in benzene. A laborious purification of the benzene for use in the MDI process chain is therefore deemed excessive and can be omitted. EP 2 246 320 A1 does not go into the non-aromatic organic compounds in the benzene which is separated from the crude product at the end of the reaction and recycled into the nitration (so-called ‘recycle benzene’).
DE 10 2009 005 324 A1 discloses that technical-grade benzene conventionally comprises 0.01% to 0.5% of low-boiling non-aromatic organic compounds (low boilers). However, the common benzene nitration processes do not use technical-grade benzene as such, but rather a mixture of recycle benzene and technical-grade benzene, so that the content of low boilers in the benzene actually used can be appreciably higher than in commercially available technical-grade benzene. By way of example, DE 10 2009 005 324 A1 discloses a value of 5% (paragraph [0007]). According to the teaching of this specification, a low boiler content of this magnitude is still not disadvantageous in the actual nitration. DE 10 2009 005 324 A1 only goes into problems with the subsequent phase separation (paragraph [0008]). A special phase separation method using a pressure-holding siphon is proposed for solving these problems.
EP 2 155 655 B1 only goes into aromatic impurities (alkyl-substituted aromatics) in the benzene.
DE-B 1 129 466 describes a process for the mononitration of technical-grade benzene, xylene and toluene comprising the customary amounts of aliphatic hydrocarbons, wherein the first runnings of unreacted aromatic obtained in the distillation of the nitroaromatic, which are rich in aliphatic impurities, are mixed with fresh aromatic and fed into the nitration, and the first runnings obtained each time are recycled as many times as desired. (Where benzene is the starting material, the first runnings of aromatic in this specification correspond to the aforementioned recycle benzene.) Those skilled in the art therefore infer from this specification the technical teaching that an increased proportion of aliphatic impurities in the aromatic to be used does not adversely affect the nitration.
EP 0 976 718 A2 discloses a process in which the off-gas is treated in an NOx absorber and then burnt There is no mention of the resulting benzene losses and the low boilers removed in this way.
It is true that the described processes of the state of the art succeed in preparing a nitrobenzene having a low content of by-products, i.e. only comprising about 100 ppm to 300 ppm of dinitrobenzene and 1500 ppm to 2500 ppm of nitrophenols, it being possible for picric acid to make up 10 wt % to 50 wt % of the nitrophenols. The processes are also distinguished by a high space-time yield.
Apart from the purity of the crude nitrobenzene, it is of decisive importance for industrial production that the preparation of the nitroaromatic be capable of being carried out in the most compact reaction equipment possible and under favourable energy conditions (such as a small excess of benzene). It is also desirable to have the most favourable raw materials possible, although they may comprise unwanted secondary components.
Impurities in the freshly introduced benzene and/or in the recycle benzene reduce the total available concentration of benzene, thereby slowing the reaction down. The reduced benzene concentration can result in the use of an excessive amount of nitric acid. This in turn increases the amount of unwanted polynitrated products and NOx gases, the latter resulting in the formation of further by-products. Critical impurities in this sense are especially aliphatic organic compounds (low boilers, cf. above). These can outgas together with NO3 gases during the reaction and hence cause further disadvantages, e.g. a poorer intermixing of the reactants and a reduced reaction volume. There was therefore a need for a method of minimizing, or ideally eliminating altogether, the content of aliphatic organic compounds in the benzene actually used in an adiabatic nitration process.